1. Field of the Invention
The present invention relates to a superconducting magnet apparatus and a magnetic resonance imaging (hereinafter referred to as MRI) apparatus using the superconducting magnet apparatus. More particularly, the present invention is concerned with an open-type superconducting magnet apparatus suitable for MRI analysis which does not give a cooped-up feeling to a subject to be inspected and an MRI apparatus using the open-type superconducting magnet apparatus.
2. Description of the Related Art
An MRI apparatus measures an electromagnetic wave emitted by hydrogen atomic nucleus spin according to a phenomenon of nuclear magnetic resonance (hereinafter referred to as NMR) and processes the electromagnetic wave as a signal to make a tomogram of the subject's body based on the hydrogen atomic nucleus density. To measure the electromagnetic wave emitted by hydrogen atomic nucleus spin, it is necessary to generate a uniform magnetic field region having high intensity and high static magnetic field uniformity as a measurement area.
Since the intensity of an electromagnetic field by the electromagnetic wave emitted by hydrogen atomic nucleus spin is proportional to the intensity of the static magnetic field in the uniform magnetic field region, it is necessary to increase the intensity of the static magnetic field in order to improve the resolution of the tomogram. Therefore, a superconducting magnet apparatus is used to generate a static magnetic field with high intensity. Furthermore, in order to eliminate distortion of the tomogram with high definition and high resolution, it is necessary to increase the magnetic field uniformity of the uniform magnetic field region. Then, a magnetic material is arranged in the superconducting magnet apparatus as a method for increasing the intensity of the static magnetic field in the uniform magnetic field region and improving the magnetic field uniformity.
An MRI apparatus, which comprises the magnetic material located in a coil vessel filled with ultracold temperature liquid helium for cooling a superconducting coil, although the magnetic material is generally arranged in the atmosphere at normal temperature, is proposed (for example, in JP-A-2001-224571 and JP-A-10-97917). Furthermore, an MRI apparatus, which comprises the magnetic material located in an intermediate-to-low temperature thermal shield system arranged for surrounding an ultracold temperature coil vessel and for shielding radiant heat from a vacuum vessel, is also proposed (for example, JP-A-2005-144132).
However, when arranging a magnetic material in the atmosphere at normal temperature, it is thought that the following two problems arise.
(1) As the temperature changes with varying room temperature, the magnetic material arranged in the atmosphere at normal temperature expands and contracts resulting in dimensional displacement. It is thought that the magnetic field uniformity changes due to this displacement.(2) For vibration when a permanent current is applied to a superconducting coil and vibration from outside, a relative displacement between a coil vessel of a superconducting magnet apparatus and a vacuum vessel for storing the coil vessel is unavoidable. In this case, it is thought that, if a magnetic material exists in the atmosphere at normal temperature, a relative displacement arises between the position of the superconducting coil in the coil vessel and the magnetic material resulting in variation of the magnetic field uniformity.
The above-mentioned problems (1) and (2) may be solved by arranging a magnetic material in the coil vessel as proposed in JP-A-2001-224571 and JP-A-10-97917. However, it is thought that the following problems arise in this case.
(3) Since the coil vessel is located in a thermal shield plate in the vacuum vessel, the magnetic material will inevitably deviate from the center axis of the superconducting coil by the gap between the thermal shield plate and the coil vessel and the thickness of the thermal shield plate and the coil vessel. Therefore, the influential sensitivity to the magnetic field on the center axis of the superconducting coil by the magnetic material decreases, making it necessary to increase the volume of the magnetic material in order to increase the sensitivity, resulting in an increase in magnet weight. In some cases, it is thought that acquisition of a required sensitivity becomes difficult even if the volume is increased.
Furthermore, when arranging a magnetic material on a thermal shield plate shown in JP-A-2005-144132, it is thought that the following problem arises.
(4) Since the heat capacity of the thermal shield plate will virtually increase, there arises a problem that an initial cooling process takes time. Furthermore, if a refrigerator for cooling the thermal shield plate stops because of power failure, the temperature of the thermal shield plate and the magnetic material rises. However, it is thought that the temperature of the magnetic material which once went up does not easily decrease and the magnetic field uniformity changes over a prolonged period of time.